Standardization of LTE (Long Term Evolution) system as the wireless communication system for 3.9-generation mobile phones has been almost completed, and recently, standardization of LTE-A (LTE-Advanced, or also called IMT-A or the like) as the 4-generation wireless communication system, which is the developed form of the LTE system, has been started.
It have been aggressively discussed in LTE-A system to eliminate dead zones where no base station can be installed and improve the received power level at the cell-edge, by deploying relay station apparatuses at arbitrary positions covered by base station apparatus.
In cases of the downlink (communication from a base station apparatus to a mobile station apparatus), a relay station apparatus may receive signals from the base station apparatus to individual mobile station apparatuses and amplify the received signals from the base station apparatus to relay the signals (L1 relay, or also called AF (Amplify-and-Forward) relay) or may decide the received signals once, re-code the obtained decoded bits and re-modulate the obtained coded bits to relay the signals (L2 relay or also called DF (Decode-And-Forward) relay). On the other hand, in cases of the uplink (communication from mobile station apparatuses to a base station apparatus), the relay station apparatus receives the signal from each mobile station apparatus and relays the signal to the base station apparatus by L1 relay, L2 relay or the like.
FIG. 16 shows one example of a mobile communication system using a relay station apparatus on the uplink. Description herein will be made on the assumption that the two mobile station apparatuses communicate with a base station apparatus by L2 relay. Further, it is also assumed that a first mobile station apparatus 1001 and a second mobile station apparatus 1002 perform data communication to a base station apparatus 1004 while a relay station apparatus 1003 is deployed between them.
First, first mobile station apparatus 1001 and second mobile station apparatus 1002 transmit transmission signals that are obtained by performing transmission processes such as coding and modulating on information bits, to base station apparatus 1004, using different radio resources (time, frequency, space) while base station apparatus 1004 receives the signals from the individual mobile station apparatuses. At the same time, relay station apparatus 1003 also receives the transmission signals transmitted by first mobile station apparatus 1001 and second mobile station apparatus 1002.
At relay station apparatus 1003, information bits are acquired from the received signal from each mobile station apparatus through reception processes such as equalization, demodulation, decoding and the like. The acquired decoded bits once again undergo transmission processes such as coding, modulation and the resultant signals are allotted to different radio resources for every mobile station, and transmitted to base station apparatus 1004.
In this way, deployment of the relay station apparatus enables the base station apparatus to receive the received signals via two routes, i.e., the wireless channel through which the signal from each mobile station apparatus directly reaches the base station apparatus and the wireless channel through which the signal reaches the base station apparatus from the relay station apparatus, hence it is possible to obtain not only diversity effect but also improve the quality of communication of mobile station apparatus at the cell-edge.
Further, non-patent document 1 for example demonstrates that when relay station apparatus 1003 relays signals from individual mobile station apparatuses, the relation station, instead of allocating different radio resources to the signals of individual mobile station apparatuses, performs network coding on the information bits of all the mobile station apparatuses by exclusive-ORing (XOR; Exclusive OR), and then transmits the resultant using a single radio resource. FIG. 17 shows the concept of this. Since the components allotted with the same reference numeral are the same as those in FIG. 16, their description is omitted.
In this figure, when the information bits transmitted from first mobile station apparatus 1001 are denoted by b1 and the information bits transmitted from second mobile station apparatus 1002 are denoted by b2, b1 and b2 are subjected to transmission processes such as error correction coding, modulation and the like, and then are transmitted as transmission signals, which are received by relay station apparatus 1003 and base station apparatus 1004. At relay station apparatus 1003, the received signals are subjected to reception processes such as demodulation, error correction decoding and the like so as to decode b1 and b2, respectively. When these signals are transmitted, the bits transmitted from individual mobile station apparatuses will not be transmitted by different radio resources, but the exclusive OR, bR between b1 and b2, represented by Ex. (1) is converted by transmission processing into a transmission signal, which in turn is transmitted.
[Math 1]bR=b1⊕b2  (1)
In Ex. (1), the symbol between b1 and b2 denotes the sign for exclusive OR operation; the operation produces 0 when two bits to be exclusive-ORed take the same value, and produces 1 when the two are different. Transmission of bR represented by Ex. (1) from the relay station apparatus makes it possible to reduce the radio resource corresponding to one transmission, hence can improve the relay efficiency and also can provide diversity effect thanks to relaying when appropriate signal processing is implanted on the base station apparatus side.
Non-patent document 2 demonstrates a specific configuration for gaining diversity from the received signal obtained by receiving the transmission signal that is generated by the thus operated relay station apparatus from the coded bits obtained by re-coding decoded bits once decoded, and the received signals having directly reached the base station apparatus from mobile station apparatuses. Description herein will be made when the number of mobile station apparatuses is 2. FIG. 18 shows one example of the mobile station apparatus.
In FIG. 18, a narrow-band single carrier that will not be affected by frequency selective fading is used for description simplicity. The mobile station apparatus includes a coding unit 1101, a modulating unit 1102, a D/A (Digital to Analog) unit 1103, a radio unit 1104 and a transmitting antenna 1105.
Information bits are error-correction coded by coding unit 1101, and modulating unit 1102 maps the information onto amplitude or phase in accordance with 0 and 1 of the input coded bits to thereby generate modulation symbols. The generated modulation symbols are converted by D/A unit 1103 from digital signals to analog signals, and the signal is up-converted to the radio frequency by radio unit 1104, and the resultant signal is transmitted from transmitting antenna 1105.
FIG. 19 shows one example of the relay station apparatus. In the relay station apparatus, the received signal, received by a receiving antenna 1111 is down-converted from the radio frequency to the baseband by a radio unit 1112, and the resultant signal is converted from analog signals to digital signals by an A/D (Analog to Digital) unit 1113.
Next, a radio resource separating unit 1114 separates the received signals into signals for individual mobile station apparatuses. The separated received signals for individual mobile station apparatuses are decomposed into bits from the modulation symbols of the individual mobile station apparatuses through demodulating units 1115-1 and 1115-2, and then are error-correction decoded by decoding units 1116-1 and 1116-2 to thereby produce estimates of information bits. The obtained information bits are rearranged in time order by interleaving units 1117-1 and 1117-2, then supplied to a changeover switch 1118. Herein, the process from demodulating unit 1115-1 to interleaving unit 1117-1 is the signal processing on the received signal reaching directly from first mobile station apparatus 1001 and the process from demodulating unit 1115-2 to interleaving unit 1117-2 is the signal processing on the received signal reaching directly from second mobile station apparatus 1002.
Changeover switch 1118 is designed to alternately switch every one bit, and the decoded bits of first mobile station apparatus 1001 and the decoded bits of second mobile station apparatus 1002 are input alternately to a coding unit 1119
In coding unit 1119, the alternately input decoded bits are error-correction coded once again by coding unit 1119, then the obtained coded bits are used by a modulating unit 1120 to generate modulation symbols. The modulation symbols are converted by a D/A unit 1121 into analog signals, which are up-converted by radio unit 1122 into the radio frequency and transmitted from transmitting antenna 1123.
In non-patent document 2, instead of using network coding by XORing, coding unit 1119 uses the original information bits and RSC (Recursive Systematic Convolutional) coding for transmitting parity bits obtained by sequentially and recursively convoluting information bits, as the network coding for a decoding process at the after mentioned receiving apparatus (base station apparatus). Here, in this specification, the coding at the relay station apparatus is called network coding. Further, the bit sequence output by network coding is called network coded bits, whereas the bit sequence output by error-correction coding is called coded bits.
FIG. 20 shows one example of the base station apparatus as the receiving apparatus. In base station apparatus 1004, the received signal is received by receiving antenna 1211, then down-converted from the radio frequency to the baseband by a radio unit 1212, then is converted into digital signals through an A/D unit 1213.
Thereafter, a radio resource separating unit 1214 separates the radio resources that have been used for communication by individual mobile station apparatuses and the radio resources that have been used for communication by relay station apparatus 1003. Here, since relay station apparatus 1003 receives the signal from each mobile station and transmits the signal after transmission processing, the signal is delayed by the time period of, at least, one transmission span. In this case, it is assumed that the received signal from each mobile station apparatus is buffered for the time period corresponding to the transmission span, to thereby realize transmission.
It is assumed in the following that the processing of the received signal from first mobile station apparatus 1001 is performed by a demodulating unit 1215-1 and decoding unit 1216-1, the processing of the received signal from second mobile station apparatus 1002 is performed by a demodulating unit 1215-2 and decoding unit 1216-2, and the processing of the received signal from relay station apparatus 1003 is performed by a demodulating unit 1215-R and decoding unit 1216-R.
The separated, received signals of individual mobile station apparatuses and relay station apparatus 1003 are input to demodulating units 1215-1, 1215-2 and 1215-R, respectively so that the received signal in modulation symbol units is decomposed into the received signal in bit units. It is noted that since log likelihood ratios (LLR) of the coded bits are often used as the received signal in bit units, LLRs will be used hereinbelow.
The obtained LLRs are input to decoding units 1216-1, 1216-2 and 1216-R and subjected to error correction. Next, the extrinsic LLRs of the information bits reformed by error correction at decoding unit 1216-1 are input interleaving unit 1218-1.
Similarly, the extrinsic LLRs of the information bits of second mobile station apparatus 1002, reformed by error correction at decoding unit 1216-2 are input interleaving unit 1218-2. In decoding unit 1216-R, the information bits transmitted from relay station apparatus 1003, consisting of the information bits of first mobile station apparatus 1001 and the information bits of second mobile station apparatus 1002, arranged alternately, are processed by error correction to calculate reformed extrinsic LLRs.
As to the extrinsic LLRs output from decoding unit 1216-R, those associated with the information bits of first mobile station apparatus 1001 are input to a deinterleaving unit 1217-1 and input to decoding unit 1216-1. Similarly, those associated with the information bits of second mobile station apparatus 1002 are input to a deinterleaving unit 1217-2 and input to decoding unit 1216-2.
The extrinsic LLRs of information bits output from interleaving units 1218-1 and 1218-2 and rearranged in time order are input alternately to decoding unit 1216-R, by the function of a changeover switch 1219. These extrinsic LLRs of information bits are used as channel information (extrinsic LLRs) for maximum a posteriori probability (MAP) estimation in decoding units 1216-1, 1216-2 and 1216-R.
This process is aimed at performing iterative decoding, by switching the parallel concatenated structure based on decoding unit 1216-1 and decoding unit 1216-R for the information bits of first mobile station apparatus 1001 and by switching the parallel concatenated structure based on decoding unit 1216-2 and decoding unit 1216-R for the information bits of second mobile station apparatus 1002. Taking into account that the information bits of relay station apparatus 1003 are formed of information bits of the individual mobile station apparatuses arranged alternately in decoding unit 1216-R, extrinsic LLRs of the information bits input from interleaving units 1218-1 and 1218-2 are changed over alternately by means of changeover switch 1219 and supplied to decoding unit 1216-R.
This process is repeated an arbitrary number of times to obtain decoded bits of each mobile station apparatus. In this way, diversity of the propagation paths through which the signals directly reach from individual mobile station apparatuses and the propagation path through which the signal reaches from relay station apparatus 1003, can be acquired by limiting the information bits by network coding and error correction coding.